The present invention relates to a plasma deposition method for forming texture on elastomer surfaces, particularly elongate elastomer surfaces such as the external surface of silicone tubing. The textured surfaces are useful on devices intended for in vivo implantation, such as within a human body.
In many medical situations, it is desirable and often necessary to implant medical devices that incorporate elongate structures. For example, elastomeric polymeric tubing, typically having a small diameter, is used in many medical applications and devices. Silicone rubber, especially cross linked silicone elastomer with silica filling, is the polymer of choice for fabricating tubing for use in many medical applications involving implantation. Other suitable elastomeric polymers include polyurethane, polyvinylchloride, polyesters and polyamides.
Implantable elongate elastomeric structures are typified by leads and catheters. Catheters prepared from elastomeric polymeric materials are used frequently in such routine procedures as the intravenous delivery of fluids, removal or drainage of urine or other fluids from compromised patients, chemical sensing using a variety of chemical transducers, monitoring cardiovascular dynamics, and treating cardiac and vascular disorders. Catheters provide the pathway to previously inaccessible body areas for both diagnostic and therapeutic procedures, thereby reducing the need for surgery. For example, double catheter systems are utilized for drug delivery or occlusion of blood flow to specific organs or tissues. In such procedures a rigid outer catheter and a buoyant, flexible inner catheter that can freely float in the blood stream are typically.
Examples of leads include cardiac pacing leads, tachycardia leads, and neurological leads. For example, a pacing lead utilizes a small diameter tubing such as less than 0.055 inch (1.40 mm) (OD) with an inner diameter (ID) of 0.35 inch (0.9 mm). In this type of lead, an elongate wire core (usually in the form of a coil) having a helical screw-in electrode at its distal end is placed inside small diameter tubing to provide a catheter-like device. The core wire is manipulated at the proximal end of this arrangement by the physician during implantation to screw the helical electrode into heart tissue and fix the lead in place.
As catheterization techniques have become more complicated, more demands placed on the performance of the catheter have increased. For instance, the paths that these catheters must take through the body are often long and tortuous, such as accessing the cranial vessels via the femoral artery. Silicone rubber tubing is especially useful for these applications because it is flexible, biocompatible, and allows for transfer of torque along its length. However, the polymeric materials from which catheters are made, such as silicone rubber, have a tacky surface upon exposure to an aqueous environment. This causes excessive friction, making placement of the catheter-like device in the body difficult. Further, these friction characteristics also make torque transfer through the tubing difficult thus, for example, making difficult the turning of the core wire which is preferably a torsion coil in the aforementioned xe2x80x9cscrew inxe2x80x9d pacing lead to screw the helical electrode into tissue.
Plasma discharge has been used on polymeric tubing to modify the surface to improve its slip characteristics, but not by creating texture on the external surface of the tubing. For example, U.S. Pat. No. 5,593,550 (Stewart et al.) is directed to a plasma process for improving the slip characteristics of polymeric tubing on its outer diameter (OD) and inner diameter (ID) surfaces. U.S. Pat. No. 5,133,422 (Coury et al.) is directed to improving the slip characteristics of polymeric tubing on its OD surface by plasma treatment in the presence of a gas selected from the group consisting of hydrogen, nitrogen, ammonia, oxygen, carbon dioxide, C2F6, C2F4, C3F6, C2H4C2H2, CH4, and mixtures thereof. U.S. Pat. No. 4,692,347 (Yasuda) is directed to plasma deposition of coatings and to improving blood compatibility on both the OD and the ID surfaces of polymeric tubing by coating it under discharge conditions in a single chamber.
Plasma reactors are well-known in the art, examples of which are described by Yasuda, H., Plasma Polymerization, Academic Press (Orlando, Fla., 1985); and d""Agostino, R., Plasma Deposition, Treatment, and Etching of Polymers, Academic Press (San Diego, Calif., 1990). Typically, such plasma reactors use wave energy (RF or microwave) to excite plasma.
In general, a plasma reactor includes a glass reaction chamber that is fitted with a vacuum exhaust, gas inlets and at least one capacitively coupled electrode. In addition, the reactor is fitted with a pressure transducer and a mass flow controller for controlling and measuring the amount of gas being introduced into the reactor. The theory and practice of radio frequency (RF) gas discharge is explained in detail in 1) xe2x80x9cGas-Discharge Techniques For Biomaterial Modificationsxe2x80x9d by Gombatz and Hoffman, CRC Critical Reviews in Biocompatibility, Vol. 4, Issue 1 (1987) pp 1-42; 2) xe2x80x9cSurface Modification and Evaluation of Some Commonly Used Catheter Materials I Surface Propertiesxe2x80x9d by Triolo and Andrade, Journal of Biomedical Materials Research, Vol. 17, 129-147 (1983), and 3) xe2x80x9cSurface Modification and Evaluation of Some Commonly Used Catheter Materials, II. Friction Characterizedxe2x80x9d also by Triolo and Andrade, Journal of Biomedical Materials Research, Vol. 17, 149-165 (1983).
Texturing of silicone surfaces has been achieved by transfer molding (photolithography) wherein a pattern is pressed into the silicone prior to curing. For example, flat stock silicone has been microtextured on one side by curing it on a microtextured glass mask or silicon wafer surface (J. Schmidt et al., Biomaterials 12, 385-389 (1991); patents describing this technology include (U.S. Pat. Nos. 5,219,361 and 5,011,494). However, transfer molding has not been used to create controlled texture on the external surfaces of elongate elastomeric structures; it is limited to planar-dimensional texturing. Microtexture on polyoxymethylene, PTFE and polyurethane surfaces has been achieved by natural ion bombardment etching (see von Recum et al., Tissue Engineering 2, 241-253 (1996)), however these surfaces are characterized by surface features having random size and distribution, rather than controlled texture comprising a deliberate array of surface features. Thermal evaporation has been used to form a random array of single-crystalline whiskers uniformly oriented with their long axes normal to the a polyimide substrate (J. Stahl et al., J. Vac. Sci. Technol. 14, 1761-1765 (1996)).
A number of patents have been reviewed in which plasma reactors are disclosed which use wave energy (RF or microwave) to excite plasma. Although not admitted as prior art, examples of plasma reactors and methods using the same can be found in the issued U.S. patents listed in Table 1 below.
The presence of surface texture may also be important for biocompatibility of medical devices, thus methods for controlling surface texture are potentially useful in controlling tissue responses to biomaterials. For example, the use of tubing having a textured surface may help limit the growth of collagenous tissue around implanted medical devices (e.g. U.S. Pat. No. 5,219,361, von Recum et al.). The ability to provide texture on the external surface of elongate elastomeric materials such as ribbons or tubing would also allow for greater manipulation of surface friction parameters.
Current texturing methods are, however, impractical for achieving texture, particularly microtexture characterized by controlled spacing of surface features, on nonplanar surfaces such as long continuous lengths of tubing surfaces, especially silicone surfaces. It is not possible to use transfer molding to form small patterns on the surface of nonplanar materials, such as long lengths of tubing. Ion beam etching does not allow for controlled or patterned spacing of surface features and, moreover, has not been demonstrated on silicone surfaces. There is, therefore, a need for a process of forming controlled microtexture on an elongate elastomeric surface.
Although this invention is generally applicable to surfaces of polymeric materials and dielectric materials, especially surfaces of elastomeric materials, it will be described herein with particular reference to silicone surfaces, more particularly nonplanar silicone surfaces such as silicone rubber tubing, but also silicone ribbons and silicone flat stock, all of which are representative of preferred embodiments of the invention.
The present invention has certain objects. That is, various embodiments of the present invention provide solutions to one or more problems existing in the prior art respecting the surface characteristics of elastomeric structures, especially elongate elastomeric structures such as tubing used in medical devices as catheters and leads. These problems include:
(1) lack of an effective method for texturing nonplanar surfaces of elastomeric structures, such as silicone tubing;
(2) growth of collagenous tissue around an implanted device having a smooth surface, such as a lead;
(3) difficulty in handling elastomeric structures having tacky surfaces, such as those fabricated from silicone; and
(4) problems with bondability of elongate elastomeric surfaces using adhesives and molding compounds.
It has been discovered according to this invention that plasma deposition can be used to accomplish microtexturing of the external surfaces of elastomeric substrates or materials, including elongate elastomeric articles such as tubing or ribbon. In one embodiment of the invention, texturing takes the form of raised elongate surface features, e.g., ridges, that typically are, but need not, be substantially perpendicular to an axis of the elastomeric material. That is, most of the ridges that make up a surface texture according to the present invention are perpendicular or nearly perpendicular to an axis of the elastomeric material, such that on the whole, the ridges that make up the surface texture appear as a group to be oriented in a direction that is perpendicular to the axis. In most cases, the surface can be examined visually and need not be the subject of quantitative measurement; the directionality of the ridges is typically readily apparent under magnification that is sufficient to distinguish them.
For example, elastomeric material can be subjected to a tension so as to stretch it prior to deposition of the surface texture; that is, plasma is deposited onto the prestretched substrate under tension to achieve surface texture. In the case of elongate material such as tubing or ribbon, the tension is preferably constant and is applied so as to stretch the tubing or ribbon longitudinally. The prestretched tubing is moved through the plasma deposition chamber in a continuous process under constant tension at a defined line speed, and raised ridges are formed that are typically roughly perpendicular to the longitudinal axis of the elastomeric material.
In the case of a planar sheet of elastomeric material, tension can be applied to stretch the sheet, for example, by pulling on opposite sides of a sheet, or by securing one side and pulling on the opposite side, thereby defining an axis extending from one side of the sheet to the opposite side of the sheet. In a preferred embodiment of the invention flat substrate is stretched along such axis prior to plasma deposition of the surface texture in a batch process, such that controlled a surface texture, comprising for example raised ridges, is formed on the prestretched surface. Alternatively, the surface texture can be plasma deposited on the unstretched surface of a planar elastomeric substrate, in which event the surface texture is typically more disordered. In addition, surface features produced by texturing unstretched planar substrates can show a radial distribution pattern around surface flaws on the substrate surface.
Surface texture formed on elastomeric substrates according to the invention can further take the form of irregularly shaped micronodules or bumps instead of, or in addition to, raised elongate structures such as ridges.
The surface features that form the textured elastomeric surfaces of the invention are micron scale; that is, as a whole, the dimensions of the features (i.e., width, height and spacing) typically fall within the range from about 0.01 micron to about 100 microns. When the surface features take an elongate form, such as ridges, the length of the surface features can be even longer; thus the term xe2x80x9cmicron scale surface featuresxe2x80x9d includes elongate surface features having widths, heights and crest-to-crest distances that fall within the range of about 0.01 micron to about 100 microns, even though their lengths may be greater than 100 microns.
Textured external surfaces of elastomeric materials of the invention are preferably characterized by controlled spacing of the surface features, such as ridges or micronodules. Control of feature spacing is achieved by manipulating the tension and/or, where applicable, line speed plasma deposition parameters to achieve the desired feature spacing. For example, in the case of elongate elastomeric materials, spacing of the features relative to the longitudinal axis of the elongate elastomeric substrate is controlled during the plasma deposition process by manipulating the tension applied to the elongate material and/or the transport line speed at which it moves through the deposition chamber. Typically the tension and line speed remain constant during a plasma deposition run, but there may be applications in which one or both of these parameters are manipulated during plasma deposition, for example, to produce tubing with different feature spacing along discrete sections of one continuous length.
Using elastomeric tubing as an example, the tubing is typically first cleaned, then spooled and placed in the plasma reactor. The tubing is continuously drawn through a glass reactor or other plasma discharge chamber (preferably thick walled glass or a suitable ceramic) which receives the tubing longitudinally from a coil or spool inside an adjacent vacuum chamber such as a bell jar. Typically, the line speed and the tension that is applied to the elongate elastomeric material are controlled by a closed loop proportional integral and derivative (PID) feedback system. This system uses an optical encoder (i.e., a laser pulse counting diode) connected to the lower drive motor to control the tubing speed, and a load cell interlocked to an upper drive motor to control tension. When the transport speed of the lower motor is established, the upper motor increases or decreases its speed to maintain the proper tension. Line speed can be monitored using a laser directed through slots cut into the change-of-direction pulley. A monomer is discharged in the glass reactor, and the tubing tension and line speed are controlled such that the monomer is deposited on the tubing to form texture on at least one external surface of the material.
The texture typically takes the form of a series of substantially parallel raised elongate structures, which appear as nodular or rounded ridges of deposited materials having crest-to-crest distances on the micron scale, the ridges being roughly perpendicular to the longitudinal axis of the elongate material as described above. That is, on the whole, the ridges that make up the surface texture appear as a group to parallel to each other, although signification deviation from true parallelism is typical and acceptable. In most cases, the surface can be inspected visually and need not be the subject of quantitative measurement; the directionality of the ridges is typically readily apparent under magnification that is sufficient to distinguish them. The height of a ridge can vary along its length.
In the case of tubing, these ridges can, but often do not, extend around the entire circumference of the tubing; however, they typically transcribe at least an are substantially along a circumference of the tubing.
Generally, any electrically non-conductive dielectric reactor chamber means which holds a vacuum will suffice as a discharge chamber. Typically, the discharge chamber is a glass chamber. In the case of tubing material, the shape of the discharge chamber is preferably tubular. Surface texture can be applied to tubing or ribbon of virtually any size diameter or width, respectively. Texture can be applied to the external surface of virtually unlimited lengths of tubing or ribbon; the only limitation is how large a spool can be fitted inside of the vacuum chamber. A typical reactor will have a capacity of 1,000 to about 5,000 feet depending on the tubing diameter or ribbon width.
The absolute size of the space relationship between the external surface of the polymer tubing or ribbon and the internal dimensions the discharge chamber in which plasma deposition is accomplished in any given instance will depend on many variables e.g., gas pressure, power applied, relative size of space in the glass tube and the size of the polymeric tubing, and so forth.
As an example, the following treatment conditions provide silicone tubing with a textured outer surface according to the invention: external diameter of the glass tube (discharge chamber) of about 1 to about 2 inches; the length of the glass tube from about 6 inches to about 26 inches;
RF power of about 30 watts to about 300 watts, applied in a continuous power mode; gas pressure in the plasma reactor of 0.010 Torr to about 5 Torr; tubing tension of about 5 grams to about 100 grams; and line speed of about 5 inches per minute to about 100 inches per minute. In any given instance, it can be readily determined empirically by varying discharge conditions and time of exposure to discharge as to what surface treatment results are obtained, then adjusting the conditions to obtain the desired result; see Example II. Moreover, operating parameters can be readily manipulated by one skilled in the art to achieve optimal reaction conditions for plasma deposition to form a textured surface on non-silicone prestretched elastomeric tubing.
Optionally, the external surface of the tubing or ribbon can be plasma pretreated prior to plasma deposition of the surface texture. Pretreatment can improve adherence of the monomer to the tubing surface when they are xe2x80x9cunlikexe2x80x9d substances, for example when fluorocarbon monomers are plasma deposited onto polyurethane surfaces, or amine-containing monomers are plasma deposited onto silicone surfaces. Typically, when the monomer and the surfaces are xe2x80x9clikexe2x80x9d substances, such as when siloxane monomers are deposited onto a silicone surface, they adhere sufficiently without plasma pretreatment. In embodiments of the invention that include plasma pretreating the elastomeric material prior to plasma deposition, the plasma reactor has at least two zones: a plasma pretreatment zone and a surface texturing zone, each having a glass reactor or plasma discharge chamber and its own set of electrodes and RF power source (see copending commonly assigned U.S. patent application Ser. No 08/923,046). For purposes of plasma pretreatment, the gas discharge process or radio frequency discharge as contemplated herein need only be such as to give rise to a plasma glow discharge which interacts with surfaces exposed thereto, such as silicone rubber, to alter same by reaction therewith. The plasma discharge apparatus thus includes a radio frequency power source, a matching network to electronically balance forward and reflected power, and cables and electrodes to deliver power to the reactor and ignite the plasma. In this embodiment the tubing or ribbon is first drawn through the plasma pretreatment zone where glow discharge electrodes are applied to the glass reactor or discharge chamber with the plasma discharge gas being inside the glass reactor. The tubing or ribbon then passes through a transition zone, then into surface texturing zone where monomer is deposited on the pretreated external surface of the tubing according to the invention.
The term xe2x80x9ctubingxe2x80x9d as used herein means an elongate cylindrical structure having one or more lumens extending along its length. Each lumen defines an inner diameter (ID) surface of the tubing. For some applications it is desirable to plasma deposit a film on the inner diameter ID surface of elastomeric tubing (see, e.g., U.S. Pat. No. 5,593,550). The present invention allows for plasma deposition on the ID surface of tubing either before or after surface texturing. If film is to be deposited on the ID surface of the tubing, the plasma reactor further includes an ID monomer deposition zone having a glass reactor or discharge chamber and its own set of electrodes and RF power source, such as that shown in U.S. Pat. No. 5,593,550 and copending commonly assigned application Ser. No 08/923,046. When plasma depositing a polymerized film on an ID surface of tubing, it is advantageous for the external surface of the tubing to be slippery since the cylindrical glass reactor used in the ID monomer deposition zone must be in very close proximity to the external surface of the tubing. If the external surface of the tubing is not plasma pretreated, then the surface texturing zone as described herein is preferably situated before the ID monomer deposition zone, since surface texturing makes the external surface more slippery. On the other hand, if the external surface of the tubing is already plasma pretreated (which improves its slip characteristics) the ID monomer deposition zone can be situated either before or after the surface texturing zone according to the invention. Preferably, however, the ID monomer deposition zone is situated after the surface texturing zone.
Also optionally, the plasma reactor can have a monomer deposition zone for additional plasma deposition onto the external surface of the tubing or ribbon either prior to or subsequent to surface texturing. This additional monomer deposition zone deposition includes a glass reactor or monomer deposition chamber and its own set of electrodes and RF power source. As in the glow discharge chamber used in the optional plasma pretreatment process, the monomer deposition chamber includes electric reactor for connection to a radio frequency power source or the like for activation of the monomer upon application of power and exposure to a monomer vapor from a monomer source. When the monomer deposition zone is situated after the surface texturing zone, the additional plasma deposited on the textured external surface of the tubing or ribbon must form a very thin layer. Preferably, the coating has a thickness of about 1 nm to about 1 xcexcm so as not to obscure or obliterate the surface texture by filling in the spaces between the ridges or nodules. Accordingly, the film that is plasma deposited in this zone preferably forms a monolayer, i.e., a layer that having a depth of about one molecule, on the external surface of the elongate elastomeric material.
The textured elastomeric material optionally incorporates a bioactive molecule into its external surface or, in the case of tubing, into its internal surface, or both. For example, the monomer deposited to create the texture can contain activated groups for subsequent covalent attachment of a bioactive monomer. Likewise, a monolayer that is plasma deposited onto the external surface of the elongate elastomeric material after texturing can include such activated groups. The internal surface of tubing can also be activated by plasma depositing onto the internal surface a monomer that contains such activated groups. Covalent attachment of the bioactive molecule to the activated internal or external surface of the elongate elastomeric material is typically accomplished by dipping the material into a solution that contains the bioactive molecule, which may itself be activated or modified so as to react with the activated internal or external surface of the material, but any convenient method for contacting the biomolecule with the activated surface of the material can be used.
If the elongate elastomeric material is externally coated after surface texturing according to the invention, for example by dipping or spraying, care must be taken to insure that the coating layer is very thin, preferably a monolayer, so as not to obscure or obliterate the surface texturing by filling in the spaces between the ridges on the external surface of the material.
Various embodiments of the present invention thus have the object of solving at least one of the problems associated with prior art devices. Texture on an external polymer surface inhibits the growth of collagenous tissue around an implanted device. In the case of bradycardia and tachyarrhythmia pacing and defibrillation leads, the control of fibrous capsule growth could significantly improve chronic removability; tissue ingrowth around conventional leads that have been implanted for some time can make removal of failed leads extremely difficult.
Moreover, in many applications it is desirable to provide a lubricious external surface on the elongate elastomeric material in order to reduce surface friction or tackiness that results when the material is exposed to aqueous environments, such as an in vivo environment. An important goal of the present invention to improve the slip characteristics of elastomeric tubing, and it has been surprisingly found that texturing the external surface of silicone tubing in accordance with the invention renders them less tacky and more manageable. Additionally, it is expected that the application of surface texture to silicone tubing in accordance with the invention will yield silicone tubing that is more manageable in the hands of the physician compared to tubing whose slip characteristics have been altered by other methods.
Surface texturing also improves surface bonding characteristics with respect to adhesives and molding compounds, which is advantageous, for example, at the terminal ends of leads.
The present method for forming controlled features on a length of elongate elastomeric material is far superior to transfer molding. Various embodiments of the plasma deposition method of the present invention provide one or more of the following advantages: (a) the method allows deposition of cross-linked siloxane coatings on the surface of silicone tubing, forming specific sized surface features; (b) the method has the ability to deposit these coatings on continuous lengths of tubing, for example tubing exceeding 1000 feet; (c) the coatings are deposited using a vacuum system and a RF plasma deposition zone; and (d) the method allows for control of the randomness or linearity of the deposited ridge features using selected process parameters.
Some of the embodiments of the invention include one or more of the following features: (a) controlled surface feature spacing, for example on the external surface of a continuous length of silicone tubing, ribbon or sheet; (b) micron scale raised nodules or ridges on an external surface of an elastomeric substrate; (c) a textured external elastomeric surface having the ability to limit or control the growth of collagenous tissue when used in vivo; (d) cross-linked siloxane coatings on the surface of silicone tubing with specific sized surface features; (e) a textured external silicone surface having improved slip characteristics.